EP0573756A2 - Method for the incineration of refuse and incinerator - Google Patents

Abstract

In the incineration of waste in waste incinerators having grate firing and co-current conduction of the flue gas, the degree of combustion of the ash is to be further improved. An increase in the primary air through the grate and thus larger grate surface areas are no longer possible. Additional appliances, such as a downstream rotary tube for ash treatment are complex to install and operate. The ash is brought into contact with hot, oxygen-containing flue gas in an ash combustion appliance (7) and is exposed to the radiant heat of the flue gas. <IMAGE>

Description

The invention relates to a method for incinerating waste according to the preamble of claim 1 and a waste incineration plant for carrying out the method according to claim 1 according to the preamble of claim 6.

When waste is burned in a grate furnace, the waste is placed on a combustion grate, conveyed through the grate over the grate with the supply of combustion air and thereby burned to ashes and flue gas. Known incineration grates are moving grates, sliding grates, traveling grates and roller grates.

Because of the inhomogeneity of waste, when it is burned on a grate, there is the problem of achieving a good burnout of both the ash and the flue gas. The burnout of ash and flue gas improves in each case through a long residence time at high temperature and through thorough mixing.

From DE-OS 33 45 867 it is known to promote the waste by mechanically transporting grids from a pre-drying zone through a combustion zone into a burnout zone. Since the flue gas originating from the individual zones flows directly into the evaporator, good burnout, at least of the flue gas, can occur from the smoldering zone, but also from the burnout zone, i.e. that is, cannot be obtained from the low temperature zones. The low temperature in the burnout zone also prevents, according to today's standards, good burnout of the ashes.

The device described in DE-OS 1 955 035 has a combustion grate and a burnout grate, both of which are supplied with compressed air. The air supply through the burnout grate leads to a better burnout of the ashes. In addition, air nozzles for blowing in part of the combustion air are set up as secondary air. Secondary air is used for secondary combustion and thus better combustion of the flue gas.

There are limits to the additional supply of combustion air, be it as primary air through additional grates or as secondary air. To minimize the CO content of the flue gas, the O₂ content of the flue gas leaving the waste incineration plant is between 6% and 10.5%. This corresponds to an excess of air of n = 1.4 to 2.0. However, the excess air to maintain high furnace temperatures should be as small as possible.

A further improvement of the burnout enables the z. B. in the DE-Z garbage and waste, 22nd Jg. (1990), pages 288-295, described direct current combustion. In this generic method and the generic waste incineration plant, the flue gas is converted to the firing material in cocurrent. that is, waste and ashes. The flue gas from the gasification area is also led through the hot combustion area. A good burnout of the flue gas is also caused by the supply of secondary air above the combustion bed, here called upper air, and at the beginning of the first flue gas draft. The intense flame radiation that acts on the ash from the start of the grate to the end of the grate improves its burnout quality. Another improvement in ash burnout, e.g. B. due to larger rust areas, is not possible due to the limited excess air. In particular, not because the primary air portion of the combustion air, also called underwind, cools the combustion grate and therefore cannot be reduced. A reduction in the proportion of secondary air would worsen the combustion of the flue gas.

To improve the ash burnout, DE-Z VDI Bildungswerke, Seminar 16.03. until 17.03.92, BW 1389, special device for treating the ash, called slag there, known as a downstream slag generator or a downstream rotary kiln. However, these devices are very complex to install and operate. In the case of a rotary tube, the turned parts working at high temperature must be serviced. In addition, the refractory lining of the rotary tube is subject to increased wear. The capacity of a rotary tube is also limited.

The object of the invention is therefore to further develop the method according to the preamble of claim 1 so that the burnout of the ash without impairing the burnout of the flue gas and without great effort.

Another object is to further develop the waste incineration plant according to the preamble of claim 6 so that it is suitable for carrying out the method according to claim 1.

The objects are solved by the characteristics of claims 1 and 6.

According to the invention, the additional burnout of the ash is brought about in a simple manner with no effort using the hot flue gas containing oxygen in a waste incineration plant. The flue gas should have temperatures of 800 ° C to 1200 ° C and an oxygen content of 6% to 11%.

The ash is conveyed via an ash burn-out device connected to the combustion grate and, above all, is kept at a high temperature by the radiation effect of the flue gas passed through the ash burn-out device. At these temperatures, the oxygen present in the flue gas is completely sufficient to burn carbon still present in the ashes. The additional burnout therefore takes place without the supply of additional combustion air. 65% to 85% of the combustion air is supplied as primary air or underwind through the combustion grate and the remaining combustion air as secondary air.

The contact between ash and hot, oxygen-containing flue gas on the ash burn-out device is ensured in that the flue gas in the conveying direction of the waste and the ash is first passed through the combustion grate and the ash burn-out device and only above the ash burn-out device into the first flue gas duct. The supply of secondary air above the combustion grate, but also at the beginning of the first flue gas flue, promotes the exhaust gas burnout by supplying oxygen and by improving the mixture due to the impulse of the air jets and thus increases the radiation effect of the exhaust gas on the ash.

A particularly good additional burn-out of the ash on the ash burn-out device is achieved if the primary air in the conveying direction of the waste is supplied to the combustion grate in at least three stages beforehand. First, an average amount of air per grate area is supplied in the drying stage, a higher amount of air per grate area in the combustion stage and finally a smaller amount of air per grate area in the burnout stage. If the waste is incinerated in these three stages, drying, incineration and burnout, the additional ash burnout on the ash burnout device leads to a further improved burnout quality.

With this method it is possible to safely reduce the carbon content of the ash to values below 1%.

According to claim 3, the dwell time of the ash on the ash burn-out device is one fifth to half the time that the waste takes to pass through the combustion grate. With shorter dwell times, only a little additional ash burnout is achieved. Longer dwell times than the specified one have hardly any additional effect.

The dwell time is initially set according to claim 4 depending on the waste composition and preferably changed during operation depending on the burnout quality of the ash and / or the flue gas. The dwell time of the ash can e.g. B. can be increased by increasing the layer height of the ash on the ash burn-out device by damming the ash.

An equalization of the layer height of the ash on the ash burnout device according to claim 5, for. B. by shaking, improves the burnout of the ashes. This is particularly advantageous when using combustion grates with low stoking effect.

In the case of waste incineration according to the invention, in which the furnace is designed as grate and direct current combustion, an ash burn-out device arranged next to the combustion grate in front of the ash removal device, according to the characterizing part of claim 6, provides an additional burn-out of the ash without impairing the burn-out of the flue gas and without great structural effort. In some cases, the combustion grate can also be made slightly smaller.

The ash burn-out device has a conveyor device for conveying the ash to the ash removal device. To support ash extraction, the conveying surface can also be inclined at a small angle to the horizontal to the ash removal.

The ash burn-out device should have a flat conveying surface. The conveying surface can also not have channels or troughs that are too deep. It is important that the entire ash is exposed to the thermal radiation of the flue gas as evenly as possible.

The ash burnout device advantageously has a width corresponding to the width of the combustion grate, so that the ash can be easily conveyed from the combustion grill to the ash burnout device.

It is free of combustion air supply devices. As a result, the entire primary air portion of the combustion air can be used to cool the combustion grate.

The arrangement of the inflow opening of the first flue gas flue over the ash burnout device, in conjunction with the combustion chamber ceiling projecting over the combustion grate, enables the hot, oxygen-containing flue gas to be passed over the ash burnout device and brought into contact with the ash for additional burnout. In addition, a space with hot flue gas is formed by this arrangement above the ash burn-out device. The heat radiation from the hot flue gas increases the temperature of the ash on the ash burnout device and thus its burnout.

The feature of claim 7 is particularly suitable for achieving the residence time of the ashes described in claim 3 on the burnout device.

A reduction in the width of the ash burnout device according to claim 8 enables a ash removal device with a smaller width.

In the formation of the ash burn-out device as a vibratory conveyor according to claim 9, the layer height of the ash conveyed above it is simultaneously equalized by shaking.

Since the conveying surface of the ash burn-out device is exposed to heat radiation and the oxygen from the flue gas, it is advantageously provided with interchangeable wear parts.

A further advantageous equipment against wear are cooling devices according to claim 11.

The invention is further explained using an example shown schematically in the drawing. Figure 1 shows a waste incineration plant and Figure 2 shows an ash burnout device.

A waste incineration plant has a combustion chamber 1 and several flue gas flues 2, in which heating surfaces 3 for the generation of steam and possibly for preheating combustion air are arranged. A flue gas cleaning system, not shown, is connected to the last flue gas flue.

The direction of flow of the flue gas is represented by arrows 4 and the direction of conveyance of the waste by arrows 5.

The combustion chamber 1 is equipped with a feed device for waste, a roller grate with five rollers 6, an ash burnout device 7 and a, e.g. B. trained as a plunger, ash removal 8.

The feed device consists of a chute 11 provided with a funnel 9 and a slide 10, which is open in the region of its base 12 to the roller grate and on the base 12 of which there is a ram feeder 13 for conveying the waste onto the roller grate. Under each of the five rollers 6 of the roller grate there is an airtight ash hopper 14 for the ashes falling through the grate. The ash funnels 14 open into a collecting device 15.

On each of the five ash funnels 14 there is a separately meterable air supply line 16, through which the respective primary air portion is supplied to the corresponding roller 6.

The ash burn-out device 7 is connected directly to the fifth roller 6 of the roller grate and ends above an ash shaft 17 opening into the ash removal device 8.

A combustion chamber ceiling 18 extending over the roller grate has approximately the shape of a pointed roof, one leg of the combustion chamber ceiling 18 protruding approximately over the fifth roller 6. This leg of the combustion chamber ceiling 18 separates the combustion chamber 1 from the first flue gas flue 2, which runs obliquely upwards in such a way that its inflow opening 19 is arranged exactly above the ash burnout device 7.

A plurality of air supply devices 20 for secondary air, also referred to as upper air, are attached to the firebox ceiling 18. Air supply devices 21 for secondary air are also located in the area of the inflow opening 19 of the first flue gas flue 2. Additional lines for returning flue gas to the various air supply devices for secondary air or primary air or to their supply lines can also be provided. In this example, at least lines are provided to the air supply devices 21 (not shown).

The ash burnout device 7 is designed as a vibratory conveyor. It has a plate 22 with a flat conveying surface, which is mounted on four elastic elements 23 and with an eccentric drive 24, for. B. a push crank drive is connected. A vibrating motor with internal unbalance can also be used as the drive. It is possible to manufacture the plate 22 from a heat-resistant material. The conveying surface is preferably provided with a layer 25 of heat-insulating material. As shown in FIG. 2, this layer 25 can consist of wear plates 26, but it could also be a stamped clay. Boundary walls 27 are attached to the sides of the conveying surface parallel to the conveying direction. In addition, the ash burnout device 7 with a cooling device, for. B. with a water cooling system with a feed line 28 and a discharge line 29.

With a length of the roller grate with five rollers 6 of 9 m, the length of the ash burn-out device 7 is preferably 2 m to 6 m. In this example it is 4 m. The width of the honeycomb grate, that of the ash burnout device 7 and that of the ash removal device 8 in this example is 3 m in each case. In particular for the combustion of waste with a low calorific value, a roller grate with six or seven rollers can be used.

In one embodiment of the invention, the width of the ash burnout device 7, starting from the width of the roller grate, takes on a width corresponding to the ash removal device 8, e.g. B. 10% to 20% smaller width, from.

In a waste incineration plant with a roller grate with a width, e.g. B. of 8 m, three ash burnout devices 7 are arranged parallel to each other in the conveying direction.

In operation, the waste, e.g. B. household waste, bulky waste, sewage sludge or industrial waste, through the funnel 9 in the chute 11 and distributed with the plunger feeder 13 on the first roller 6. By rotating the rollers 6 at about three to five revolutions per hour, the waste is conveyed through the roller grate, possibly with the supply of primary air and from recirculated flue gas, and burned to ash and flue gas.

The flue gas produced during the combustion is supplied with secondary air and possibly recirculated flue gas above the roller grate through the air inlets 20 in the combustion chamber ceiling 18 and secondary air and / or recirculated flue gas in the area of the inflow opening 19 of the first flue gas duct 2 through the air supply 21.

The excess air n of the total combustion air supplied is 1.4 to 2.0, which corresponds to an oxygen content of 6% to 11% of the flue gas leaving the flue gas flues.

In this example, an excess air of n = 1.6 and a primary air share of 80% are set for the combustion of household waste with a calorific value of 10,000 kJ / kg.

The secondary air portion of 20% is supplied through the air inlets 20 in the combustion chamber ceiling 18. Returned flue gas is supplied through the air supply lines 21 in the region of the inflow opening 19 of the first flue gas train.

The primary air is supplied in a drying stage, a combustion stage and a burnout stage.

In the drying stage, an average amount of air per grate surface is created by the first roller 6, in the combustion stage by a second and third roller 6, a larger amount of air per grate surface by a factor of 1.1 to 2 and in the burnout stage by the fourth and fifth roller 6 by a 0.1 to 0.9 lower amount of air per grate area.

The combustion and burnout stages are further subdivided.

The supply of primary air through the individual rollers 6 is set to the values listed in the following table: roller Share of primary air in% factor Drying stage 1 20th 1 Combustion stage 2nd 35 1.75 3rd 25th 1.25 Burnout stage 4th 15 0.75 5 5 0.25

The grading of the supply of primary air, but also the proportion of primary air, is regulated depending on the combustion behavior, combustion chamber and / or flue gas temperatures and the pollutant content of the flue gas.

With the staged supply of primary air, the waste is first dried and possibly carbonized before it is burned and finally burned out.

The residence time of the waste and the ashes on the roller grate is 20 minutes to 40 minutes. The ash is then conveyed through the ash burn-out device 7.

The supply of secondary air generates evenly hot and afterburned flue gas through the air inlets 20 in the combustion chamber ceiling 18 above the roller grate. Due to the shape of the combustion chamber ceiling 18 and the arrangement of the inflow opening 19 of the first flue gas duct 2 above the ash burnout device 7, this flue gas is guided via the roller grate to the ash burnout device 7 and via the ash burnout device 7 into the first flue gas flue 2. As a result, the ash lying on the ash burn-out device 7 is brought into contact with the hot, oxygen-containing flue gas and exposed to the heat radiation of the flue gas flowing into the first flue gas train 2, which is further mixed by the supply of recirculated flue gas, and burned out to a carbon content of less than 1%.

The dwell time of the ash on the ash burn-out device 7 is in the above. Household waste is set to 10 minutes to 30 minutes. The conveying speed of the ash and thus its dwell time on the ash burn-out device 7 is adjusted during operation by adjusting the shaking frequency, i.e. that is, by adjusting the angular velocity of the drive 24, or by adjusting the angle of the conveying surface to the horizontal depending on the burnout quality of the flue gas or the ash. This sets the layer height of the ash on the ash burn-out device 7. At the same time, the layer height is evened out by shaking the vibratory conveyor.

Claims (11)

Waste incineration method, wherein
the waste is placed on a combustion grate,
conveyed over the combustion grate and
is burned to ashes and flue gas by supplying primary air through the combustion grate,
the flue gas is guided over the combustion grate in the direction of conveyance of the waste and ashes and then cooled in at least one flue gas flue, and
Secondary air is fed to the flue gas above the combustion grate and / or at the beginning of the first flue gas flue,
characterized in that
the ash is then conveyed to the combustion grate via an ash burn-out device (7), and
the flue gas is passed via the ash burnout device (7) and above the ash burnout device (7) into the first flue gas flue (2),
the ashes are further burned out in the presence of the hot, oxygen-containing flue gas without additional combustion air. A method according to claim 1, characterized in that the primary air is supplied to the combustion grate in at least three stages, a drying stage, a combustion stage and a burnout stage. Method according to claim 1 or 2, characterized in that the dwell time of the ash on the ash burn-out device (7) is one fifth to half the time for passing the combustion grate. Method according to one of claims 1 to 3, characterized in that the dwell time of the ash on the ash burn-out device (7) is set depending on the waste supplied and / or on the burn-out quality of the ash and / or the flue gas. Method according to one of claims 1 to 3, characterized in that the layer height of the ash on the ash burn-out device (7) is made more uniform. Waste incineration plant for performing the method according to claim 1 with a grate furnace and at least one flue gas flue, wherein
the grate firing has a feed device, at least one combustion grate and a deasher,
at least one combustion air supply device is arranged under the combustion grate, and
a firebox ceiling protrudes over the combustion grate,
characterized in that
an ash burnout device (7) is then arranged on the combustion grate in front of the ash removal device (8), wherein
the ash burnout device (7) is provided with a conveying device, has an approximately flat conveying surface with a width corresponding to the width of the combustion grate, and is free of supply devices for combustion air, and
the inflow opening (19) of the first flue gas flue (2) is arranged above the ash burnout device (7). Waste incineration plant according to claim 6, characterized in that the length of the ash burnout device (7) is one third to two thirds, in particular one fourth to half the length of the combustion grate. Waste incineration plant according to claim 6 or 7, characterized in that the width of the ash burn-out device (7), starting from the width of the combustion grate, decreases in the direction of conveyance of the ash. Waste incineration plant according to one of claims 6 to 8, characterized in that the ash burn-out device (7) is designed as a vibratory conveyor (22, 23, 24) Waste incineration plant according to one of claims 6 to 9, characterized in that the conveying surface of the ash burn-out device (7) has replaceable wear parts (26). Waste incineration plant according to one of claims 6 to 10, characterized in that the ash burnout device (7) has cooling device (28, 29).

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